Only add assertion messages on failure.

Adding assertion messages when missing was slowing down the code
because of the repeated throwing at catching of exceptions.  Now
only add a stack trace if the assertion will fail.

Issue #288

Change-Id: Ic4ec3180df0178f1e3e0b25534d9e5bb018707a4

lib/debug/asserts.js

@@ -58,7 +58,7 @@ if (shaka.asserts.ENABLE_ASSERTS) {
 
   shaka.asserts.assert = function(test, opt_message) {
     var message = opt_message;
-    if (!message) {
+    if (!test && !message) {
       // IE11 does not display any stack information on assertions.  So if
       // there is no message, create one from the stack.
       try {